There is increasing evidence suggesting that coastal dunes evolve as a consequence of discrete transport events. In particular, sediment input by wind from the beach to the adjacent foredunes is intermittent both at the instantaneous and medium scales. Knowledge of sand transport by wind is essential for sediment budget calculations but information about the nature of aeolian sand transport events at the meso-scale in coastal areas is surprisingly limited. The lack of a comprehensive view on the dynamics of aeolian sediment transport, beyond the duration of short-term experiments, is a major limitation to providing managers and planners with an appropriate set of tools for decision-making. The main objective of this study was to characterize the magnitude and frequency of aeolian transport events on a beach and to assess their role in supplying sediment for foredune building. Long-term monitoring of the beach and foredune was carried out at Greenwich Dunes, Prince Edward Island (PEI) National Park, Canada. The foredune ranges in height from 6 to 10 m, and the beach is generally 30-40 m wide. Wet days number 130 to 160 a year in PEI, with snow days accounting for 30% of them and about one third of the precipitation comes in the form of snow. Winter storms from the NE have the potential to deliver large volumes of sediment to the dune area but they may also be accompanied by heavy rain or snowfall and large storm surges. The long term monitoring station was design to permit the acquisition of continuous records of wind and beach characteristics that can be coupled with sand transport measurements over periods of months. It consisted of three 8-megapixel digital single lens reflex (SLR) cameras mounted on the top of a 6 m high mast on the crest of the 8 m foredune. They were controlled by a Mumford Time Machine TM programmed to take pictures every hour. The images were rectified and used to extract numerical information on beach surface moisture, snow and ice cover, vegetation cover, and shoreline location. Wind speed and direction were measured using a Windsonic 2-D sonic anemometer mounted at the top of the mast. Sediment transport and deposition were measured using saltation probes, erosion-deposition pins, and survey technique. A Geodatabase (PEI GDB) built with ArcCatalog was developed to store and manage time series. There were 185 wind events from September 2007 to May 2008. About 60% of wind events did not result in aeolian sediment movement. Surprisingly, the highest wind speeds did not result in any net transport into the dunes because of the limitations imposed by ice/snow cover, moisture, and short fetches. Despite extreme winds during intense storms, such events often induced wave scarping rather than aeolian sediment input to the foredunes. When sand transport was active on the beach, the flux magnitude was regulated by a combination of factors including angle of wind approach, fetch distance, moisture content, and duration of the wind event. Only three wind events accounted for approximately 75% of the total transport delivered to the foredune in 9 months. The remainder was accounted for by a small number of events of lesser magnitude. The peak of aeolian transport was centered between small and medium magnitude wind events generating low to medium magnitude transport. Information about the nature of transport events can provide a route to enhance our ability to model meso-scale sand supply to the foredune. The probability of a simple positive correlation between wind event magnitude and resulting transport towards the foredune decreases as wind speed increases beyond a certain threshold because factors such as wave inundation play an increasing role in preventing sediment transport across the beach. This has implications for modeling foredune evolution, and beach and dune interaction, and introduces differences between the magnitude and frequency of aeolian transport events in the coastal environment compared to those in deserts and rivers.
|Published - 2010
|6th Young Coastal Scientists and Engineers Conference - University College London, London, United Kingdom
Duration: 29 Mar 2010 → 30 Mar 2010
|6th Young Coastal Scientists and Engineers Conference
|29/03/10 → 30/03/10